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Proto Krakatau

Historical and archaeological
evidence now suggests that the European Dark Ages might have been triggered by
a massive volcanic eruption. The global consequences to humanity of a similar
super eruption are much more complex and far reaching than has ever been
considered according to Ken Wohletz. Such an event can fundamentally and
permanently change human history. There are numerous potentially active
calderas that can erupt in the near future with a similar magnitude. – B.H.

Modern history has
its origins in the tumultuous 6th and 7th centuries. During this period
agricultural failures and the emergence of the plague contributed to: (1) the
demise of ancient super cities, old Persia, Indonesian civilizations, the Nasca
culture of South America, and southern Arabian civilizations; (2) the schism of
the Roman Empire with the conception of many nation states and the re-birth of
a united China; and (3) the origin and spread of Islam while Arian Christianity
disappeared. In his book, Catastrophe An Investigation into the Origins of the
Modern World, author David Keys explores history and archaeology to link all of
these human upheavals to climate destabilization brought on by a natural
catastrophe, with strong evidence from tree-ring and ice-core data that it
occurred in 535 AD. With no supporting evidence for an impact-related event, I
worked with Keys to narrow down the possibilities for a volcanic eruption that
could affect both hemispheres and bring about several decades of disrupted
climate patterns, most notably colder and drier weather in Europe and Asia,
where descriptions of months with diminished sun light, persistent cold, and
anomalous summer snow falls are recorded in 6th-century written accounts.
Writings from China and Indonesia describe rare atmospheric phenomena that
possibly point to a volcano in the Indonesian arc. Although radiocarbon dating
of eruptions in that part of the world are spotty, there is strong bathymetric
and volcanic evidence that Krakatau might have experienced a huge caldera
eruption. Accordingly, I encouraged a scientific expedition to be led by
Haraldur Sigurdsson to the area. The expedition found a thick pyroclastic
deposit, bracketed by appropriate radiometric dates, that suggests such a
caldera collapse of a “Proto-Krakatau” did occur perhaps in the 6th century.
Bathymetry indicates a caldera some 40 to 60 km in diameter that, with collapse
below sea level, could have formed the Sunda Straits, separating Java from
Sumatra, as suggested by ancient Javanese historical writings. Such a caldera
collapse likely involved eruption of several hundred cubic kilometers of
pyroclastic debris, several times larger than the 1815 eruption of Tambora.
This hypothetical eruption likely involved magma-seawater interaction, as past
eruptions of Krakatau document, but on a tremendous scale. Computer simulations
of the eruption indicate that the interaction could have produced a plume from
25 to >50 km high, carrying from 50 to 100 km3 of vaporized seawater into
the atmosphere. Although most of the vapor condenses and falls out from low altitudes,
still large quantities are lofted into the stratosphere, forming ice clouds
with super fine (<10 micrometer) hydrovolcanic ash. Discussions with global
climate modelers at Los Alamos National.

Laboratory led me to preliminary calculations that such a plume of ash
and ice crystals could form a significant cloud layer over much of the northern
and southern hemispheres. Orders of magnitude larger than previously studied
volcanic plumes, its dissipation and impact upon global albedo, the tropopause
height, and stratospheric ozone are unknown but certainly within possibilities
for climate destabilization lasting years or perhaps several decades. If this
volcanic hypothesis is correct, the global, domino-like effects upon epidemics,
agriculture, politics, economics, and religion are far-reaching, elevating the
potential role of volcanism as a major climate control, and demonstrating the
intimate link between human affairs and nature.

Krakatau?

Tree-ring data from Keith Briffa
(CRU, Univ. of East Anglia), corroborated by European data compiled by Mike
Baillie (Queen’s Univ., Belfast), shows clear evidence of a 535 AD climate
perturbation, and it is now known worldwide. The origins of this event are
likely to have been volcanic since ice core from Greenland and Antarctica show
sulfuric acid spikes during this time interval, and for the Byrd core
(Antarctica), it is the largest in the last 2000 years. David Keys and I worked
with Claus Hammer (Niels Bohr Institute) to reinvestigate the GRIP core from
Greenland that Clausen et al. (1997) had already identified.

Although asteroid/comet impact
remain as potential causes, I focus on a volcanic source located near the
equator. Of over 100 potential equatorial volcanoes considered, I found best
corroborating evidence in Indonesia, where 6th Century geo-political
discontinuity is well documented. Tephra dates are very useful, but there can
be pitfalls. For example, some published dates for Rabaul that looked like a
fit turned out to be erroneous. It was a translation of the Javenese “Pustaka
Raja Purwa” (The Book of Ancient Kings) that alerted David Keys and I to a
massive eruption of Krakatau during the 338th year of the Shaka Calendar, which
is known to likely be misaligned to the western calendar date of AD 416. We
spent considerable time identifying and translating text that could be
demonstrated as not having been “contaminated” by post 1883 writings, and in
that text is the ancient Javanese tradition of the separation of Java from
Sumatra in a cataclysm that fits a description of a large explosive eruption.
In other words, a caldera collapse that produced the modern Sunda Straits.

A pre-1883 British Admiralty chart
of Sunda Straits shows shallow (~10 m) depths of sea floor in straits, and
islands of Krakatau, Bezee, Sebooko, Thwart way, and mountains near Katimbang
(Mt. Rajah Bassa); these islands may be vestiges of volcanic vents surrounding
the flanks of Proto-Krakatau, the predecessor of the present Krakatau
(Krakatoa) volcano. Connecting these vents outlines a caldera with a surface
diameter of ~50 km centered in the Sunda Straits about 20 km NE of present day
Krakatau.

Drawing a west-to-east cross
section of hypothetical caldera collapse, shows how Sunda Straits might have
formed during the 6th century eruption of Proto Krakatau.

Reconstructing the Eruption

If one assumes that the average
amount of collapse over the entire caldera area averaged ~100 m, then the
volume of collapse was approximately 200 km3. This volume is a rough estimate
of the amount of magma evacuated from the magma chamber at a depth of several
km below the Sunda Straits, a chamber that might have held over a 2000 km3 of
magma at the time of eruption (a cylindrical body 50 km in diameter and several
km thick).

The 6th century eruption likely
started with widespread tumescence of the ground over an area of a thousand
square kilometers around Proto Krakatau, occurring over a period of years and
reaching a magnitude of several meters or more near Proto Krakatau. This rising
of the ground was likely to have proceeded so slowly that people in the area
might not have noticed it, but they would have noticed the ever increasing
occurrence of small earthquakes that perhaps were felt every few weeks in the
year before the giant eruptions and reached nearly continuous shaking in the
weeks before the eruption.

The following
illustrations were generated by the volcanic eruption simulator, Erupt3, and
they depict cross-sectional views of the Sumatra-Java island arc structure,
built up by volcanism over thousands of years, consisting of layers of lava,
pumice, and ash (red, magenta, and green colors). Proto-Krakatau is shown as a
cone-like structure forming a land area connecting Sumatra to the west and Java
to the east.

Analyzing the Eruption

While the preceding eruption
simulations give a good qualitative picture of how the eruption progressed, we
desire more detailed information, specifically regarding the physical
parameters of the eruption. Some of these parameters can be constrained by
considering the scale of the eruption portrayed above, while others must be
calculated. Using the results of supercomputer simulations that solve
mathematical equations that express the physical behavior of the eruption, we
obtain results that are useful for atmospheric and sound wave models.

Eruption Duration: From the size
of the assumed caldera, we showed above that ~200 km3 of magma was erupted. For
eruptions of this magnitude, scaling of smaller historical eruptions indicates
mass discharge rates of 1 billion kg per second or more. This flux is equivalent
to one-one thousandths of a km3 per second or 3.6 km3 per hour. All other
parameters being equal, the eruption would have taken at least 34 hours, but
owing to waxing and waning fluxes during the eruption, the cataclysmal parts of
the eruption might have lasted over 10 days. In fact, such eruptions might
occur in day-long pulses, occurring over a period of years.

Eruption Products: Most of magma
was fragmented by the tremendous forces of the eruption into pieces of pumice
and ash. Especially during the Phreatoplinian eruption, ash was the preferred
form of expelled magma. This ash is composed of tiny fragments of rock and
glass shards, ranging from about 1 micrometer to a couple of millimeters in
diameter. During the Phreatoplinian eruption, as much as 50% of this ash could
have been composed of fragments less than 50 micrometers in diameter. Such tiny
fragments have exceedingly long resident times in the atmosphere, able to be
kept aloft for months by normal atmospheric turbulence. Assuming that 75% of
the total 125 km3 of magma were involved with the Phreatoplinian eruption,
perhaps as much as 30 km3 of fine ash particles were put into global
circulation. Sulfur from the magma likely condensed on ash particles as
sulfuric acid droplets, but its abundance is not known.

Water Vapor: Along with the
pumice and ash, a lot of water was vaporized and injected into the atmosphere
and stratosphere, especially during the Phreatoplinian phase of the eruption,
perhaps volumetrically the most important of the cataclysmal phases. For
optimum water vaporization, molten magma contains enough heat to vaporize an
approximately equal volume of water. Assuming that 75 of the total 125 km3 of
magma interacted with water, 75 km3 were vaporized, which upon expansion to atmospheric
pressure may have occupied nearly 100,000 km3 of the atmosphere. To be sure,
much of this vapor would have condensed an fallen out as ash-clogged rain with
in hours of the eruption, but perhaps as much as half of it was carried around
the world by stratospheric winds. This vapor would have condensed to form ice
crystals, and these ice crystals would disperse in the rarified air to form
stratus clouds, darkened by entrained ash. Along with the water vapor from the
sea chlorine is also carried into the stratosphere, a component having an
important effect on stratospheric chemistry.

Jet and Plume
Structure: The jet of ash, pumice, and gas emerged from the vent at supersonic
speeds of 650 m/s but decelerated to slow buoyant rise speeds of several tens
of meters per second after reaching 10 to 15 km into the atmosphere. The
buoyant plume had sufficient energy to continue rising to nearly 50 km before
it became neutrally buoyant and began spreading laterally. The width of the jet
and plume during the Plinian eruption phase may have ranged from several km at
its base to many as many as 40 km near its apex, just below the level where it
spread out as a giant anvil-shaped cloud. Ash concentrations within the plume
decreased upwards because of admixture of ambient air into the plume as it
rose, but these concentrations probably stayed at levels of 1 part in a million
by volume. This is a very conservative estimate of total plume volume over the
course of the eruption, giving a value of well over 100 million km3 and likely
reaching several orders of magnitude.

Summary:

Magnitude

Collapse of 100 m over 50-km diameter caldera can involve
eruption of ~200 km3 of magma